Making matters worse, polyolester (POE) oils are commonly used in modern HVAC/R systems operating with hydrofluorocarbon refrigerants, replacing mineral oils which were commonly used with chlorinated refrigerants. POE is formed by polymerization reactions between alcohols and carboxylic acids, and is susceptible to hydrolysis in the presence of water. Water can react with synthetic oils such as POE, PVE and PAG to form unwanted chemical reaction products including acids. In the case of POE oil, water will break the ester bonds to form organic acids and alcohols. These organic acids have been implicated in corrosion and sludge formation in HVAC/R systems.
Moisture can enter the refrigeration system through improper evacuation procedures or by the service technician or manufacturer inadvertently introducing oil that is saturated with moisture due to extended exposure of the oil to the ambient air (which usually contains very high levels of moisture relative to the limits in a refrigeration system).
Despite all attempts to keep the HVAC/R system as dry as possible, moisture can still enter the system and be absorbed into the oil. Although POE oil is hygroscopic and will readily absorb moisture upon exposure, even mineral oils can absorb moisture albeit their saturated moisture levels being much lower about 25 Parts Per Million (PPM) for mineral oil compared to 2,500 for POE oil, 6,500 for PVE oil and 10,000 PPM for PAG oil. Even with mineral oil's much lower saturation moisture levels, however, it is not always desirable to change out the oil and the refrigerant in the event of moisture or acid contamination.
While the common filter-drier used in a typical vapor compression system will remove acid and moisture from the HVAC/R system (as long as its absorption capacity has not been reached) the filter-drier is typically placed in the liquid line or occasionally also in the vapor line. Therefore, a filter can only remove some portion of the acid and moisture that passes through the filter-drier, the acid and moisture being carried along with the refrigerant and oil mixture. The majority of the acid and moisture will, however, still reside in the oil. The moisture will remain in the oil until the oil is saturated with water, which can mean a significant amount of moisture being present in the case of synthetic lubricants or oils. The acid tends to be dissolved in that trapped moisture. As above noted, mineral oil becomes saturated with moisture at 25 PPM; however, the synthetic oils have saturations limits that are at least 2,500 PPM. Since most of the refrigerant and oil mixture flowing through the liquid or vapor lines is refrigerant with the oil concentration being only a few percent, the filter-drier is ineffective at removing the acid and moisture from the majority of the oil that remains in the oil sump of the compressor, or that remains trapped on any surfaces, before the acid and moisture in the system has had sufficient time to react and create problems in the system that lead to the destruction of the HVAC/R unit.
U.S. Pat. No. 5,770,048 teaches the introduction of an alcohol-containing composition into an HVAC/R system to accelerate the separation of the acid composition from the system and thereby speed the delivery of the acid to the filter drier where it is removed. This formulation unfortunately had no effect on removing moisture trapped in the system and accelerating its removal. We have now developed a chemical approach to remove the moisture as well as the acid, thereby dramatically improving upon the formulation taught in U.S. Pat. No. 5,770,048. In particular, we have discovered that the novel combination of known acid removal compound with our moisture removal composition does not form any unwanted byproducts, with each compound operating without damaging the efficacy of the other.
Therefore, it is highly advantageous to be able to remove acid and moisture quickly from the refrigerant and the oil without interrupting the operation of the HVAC/R system. Additionally, it is desirable to have a way of protecting against future moisture contamination.
One way to remove moisture from an HVAC/R system is by the introduction of a chemical additive to the system that consumes water via a chemical reaction. U.S. Patent Application No. 2014/0165625 A1 teaches the use of hydrolytic orthoesters drying agents which are introduced into an operating HVAC/R system. Orthoesters are specifically emphasized as hydrolytic moisture scavenging additives and only broadly suggests the use of hydrolytic drying agents (e.g. acetals, epoxides and carbodiimides). While orthoesters, specifically orthoformates, remove water from refrigeration oils, they can react to form an ester and an alcohol, which in and of itself would be acceptable; however, esters can then be hydrolyzed further to form acid (e.g. formic acid) under certain conditions which is highly undesirable in HVAC/R system. Therefore, an improved moisture removal additive is desperately needed for HVAC/R systems that would not potentially hydrolyze to form acids since acids are responsible for corrosion, compressor burn-outs and continued oil/refrigerant degradation.
It is also well known in the art to use silanes as moisture scavengers, however the use of silanes can interfere with the action of silane-based leak sealing additives and therefore are not optimal. That is, the silanes used for moisture removal are selected to form soluble polymers that stay dissolved in the oil whereas silanes used to seal leaks are designed to form highly cross-linked insoluble polymers at the leak site. The moisture removing silanes could thus prevent cross-linking of the leak sealing chemicals and reduce their effectiveness.
To resolve the problem of undesired by-products of the moisture drying reaction, we discovered that a drying formulation for removing moisture from the refrigerant and oil in an operating HVAC/R systems. The drying agent capitalizes on the hydrolysis of 2,2-dimethoxypropane to remove the moisture from the refrigerant and oil by consuming the water molecules and advantageously producing methanol as well as producing acetone which does react any further in the HVAC/R system. This prevents the problem that occurs when using orthoformates, namely the buildup of acid due to over-hydrolysis of orthoformates that are used in HVAC/R drying formulas such as the above-referenced U.S. Patent Application No. 2014/0165625 A1.
As stated earlier, while U.S. Patent Application No. 2014/0165625 A1 broadly suggests the use of hydrolytic drying agents including acetals, and although 2,2-dimethoxypropane is also known as acetone dimethyl acetal, the prior art appears not to have recognized, as we did, that it is a far superior drying agent that will not form unwanted harmful reaction products. Furthermore, the prior art appears not to have realized that this compounds reaction with the water molecules produces methanol which will liberate the trapped acid from the oil allowing it to vaporize and travel to the filter drier, thereby accelerating the removal of acid from the system. We have found that United States Air Force Contract AF 33(616)-7006 taught the use of 2,2-dimethoxypropane, anhydrous hydrazine, trimethyl orthoacetate, trimethyl orthoformate, trimethyl orthovalerate as well as two silanes only to prevent fuel filter icing in mineral-oil based aviation fuels. However, this document provides no guidance for use of 2,2-dimethoxypropane in HVAC/R systems nor suggests their superiority by not further hydrolyzing to acid, as is possible with orthoesters.